Re: [PATCH][RFC] fix kernel BUG at mm/migrate.c:719! in2.6.26-rc5-mm3

[Daisuke Nishimura]

On Wed, 18 Jun 2008 10:13:49 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> On Tue, 17 Jun 2008 16:35:01 +0900
> Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:
> 
> > This patch also fixes a race between migrate_entry_wait and
> > page_freeze_refs in migrate_page_move_mapping.
> > 
> Ok, let's fix one by one. please add your Signed-off-by if ok.
> 
Agree. It should be fixed independently.

Signed-off-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>

> This is a fix for page migration under speculative page lookup protocol.
> -Kame
> ==
> In speculative page cache lookup protocol, page_count(page) is set to 0
> while radix-tree midification is going on, truncation, migration, etc...
> 
> While page migration, a page fault to page under migration should wait
> unlock_page() and migration_entry_wait() waits for the page from its
> pte entry. It does get_page() -> wait_on_page_locked() -> put_page() now.
> 
> In page migration, page_freeze_refs() -> page_unfreeze_refs() is called.
> 
> Here, page_unfreeze_refs() expects page_count(page) == 0 and panics
> if page_count(page) != 0. To avoid this, we shouldn't touch page_count()
> if it is zero. This patch uses page_cache_get_speculative() to avoid
> the panic.
> 
> From: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> ---
>  mm/migrate.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> Index: test-2.6.26-rc5-mm3/mm/migrate.c
> ===================================================================
> --- test-2.6.26-rc5-mm3.orig/mm/migrate.c
> +++ test-2.6.26-rc5-mm3/mm/migrate.c
> @@ -243,7 +243,8 @@ void migration_entry_wait(struct mm_stru
>  
>  	page = migration_entry_to_page(entry);
>  
> -	get_page(page);
> +	if (!page_cache_get_speculative())
> +		goto out;
>  	pte_unmap_unlock(ptep, ptl);
>  	wait_on_page_locked(page);
>  	put_page(page);
> 

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